If they don't have immediate cash for U.S. customer withdrawals, is there also delay for those customers' BTC purchases to be completed?
You don't make *real* BTC purchases from Mt. Gox with *real* USD you make purchases of Mt. Gox BTC with Mt. Gox USD (like all the exchanges) - so as long as you have sufficient Mt. Gox BTC/USD funds you can trade.
In short:
*real* BTC -> Mt. Gox BTC easy (wait for confirmations)
*real* USD -> Mt. Gox USD easy (but slower than above)
Mt. Gox BTC -> Mt. Gox USD easy (and instant)
Mt. Gox USD -> Mt. Gox BTC easy (and instant)
Mt. Gox BTC -> *real* BTC easy (although sometimes delays)
Mt. Gox USD -> *real* USD hard (can expect to wait months)